1. Field of the Invention
The present invention relates to a method of exposure adapted for use, for example in the manufacture of a semiconductor device, in projection exposure of a mask pattern on a photosensitive substrate.
2. Related Background Art
In the manufacture of a semiconductor device, a liquid crystal display device or the like by a photolithographic process, there is employed a projection exposure apparatus for projecting the image of the pattern of a photomask or a reticle (hereinafter collectively called reticle) through a projection optical system onto each of the shot areas on a wafer (or a glass plate) coated with photosensitive material. For such projection exposure, there is recently employed the exposure apparatus of step-and-repeat method, particularly the exposure apparatus of reduced projection, or stepper. In such apparatus the wafer is placed on a two-dimensionally movable stage, and is stepped by said stage whereby the shot areas on the wafer are exposed in succession to the image of the pattern on the reticle.
In such projection exposure apparatus, there is generally employed a projection optical system of a high numerical aperture because of the requirement for a high resolving power, so that the depth of focus of the projected image obtained by the projection optical system becomes smaller in reciprocal proportion to the square of the numerical aperture. Therefore, in order to position each shot area of the wafer within the depth of focus with respect to the focal plane of the projection optical system, the projection exposure apparatus is conventionally equipped with the following auto focusing mechanism. Said mechanism is composed of a focus position detecting sensor for detecting the amount of difference between the focus position (corresponding to the axial position of the projection optical system) at a predetermined measuring point in each shot area on the wafer and the focal plane of the projection optical system (said sensor being hereinafter called AF sensor), and a servo system for maintaining said amount of difference within a tolerance range.
In such projection exposure apparatus, the exposure is conducted in the following steps of (1) positioning a predetermined shot area on the wafer at the position of exposure, (2) focusing, for example, the center of the shot area (shot center) to the focal plane by the servo system, utilizing the result of detection by the AF sensor, and (3) exposing the shot area to the image of the reticle pattern.
Consequently, the auto focusing mechanism has been designed to focus, for example, the shot center of each shot area to the focal plane.
In the above-explained prior art, the area to be focused by the auto focusing mechanism is fixed in each shot area. For this reason, the area requiring most exact focusing in each shot area may be positioned outside the depth of focus of the focal plane of the projection optical system. For example, if the reticle pattern contains circuit patterns of different line widths, the area requiring most exact focusing within each shot area on the wafer is the area where the image of circuit pattern of the narrowest line width is projected. Such area, however, may be different from the area focused by the auto focusing mechanism, for example the shot center.
Also with the recent progress in the level of integration of the semiconductor device, the wafer in the device manufacturing process tends to include significant step differences instead of being flat. For this reason, if the relative positional relationship between the optical axis of the AF sensor and that of the projection optical system is different among plural projection exposure apparatus, there is encountered a drawback that the focal position becomes different from apparatus to apparatus.